A direct methanol fuel cell (DMFC), like an ordinary battery, provides dc electricity from two electrochemical reactions. These reactions occur at electrodes to which reactants are continuously fed. The negative electrode (anode) is maintained by supplying a fuel such as methanol, whereas the positive electrode (cathode) is maintained by the supply of oxygen or air. When providing current, methanol is electrochemically oxidized at the anode electro-catalyst to produce electrons, which travel through the external circuit to the cathode electro-catalyst where they are consumed together with oxygen in a reduction reaction. The circuit is maintained within the cell by the conduction of protons in the electrolyte.
A DMFC system integrates a direct methanol fuel cell with different subsystems for instance for the management of water, fuel, air, humidification and thermal condition of the system. These subsystems are aimed to improve the overall efficiency of the system, which typically suffers from kinetic constraints within both electrode reactions together with the components of the cell stack. For instance, water management issues are particularly critical for a polymer electrolyte membrane (PEM) stack used for a DMFC system. On the one hand, the DMFC stack must maintain sufficient water content to avoid membrane dehydration and to avoid dry out of the cathode catalyst layer. Membrane dehydration increases the membrane resistance while a dry cathode lowers the oxygen reduction activity of the platinum catalyst; both reduce DMFC stack performance. On the other hand and more common in practice, water management problems in a DMFC stack are more often associated with excess water in the stack rather than dry out. Excess water can interfere with the diffusion of oxygen into the catalyst layer by forming a water film around the catalyst particles (flooding). In traditional DMFC systems the fuel cell stack water content is managed by controlling the stack temperature and air flow rate by for instance an air compressor system and an air-to-air condenser. However, such systems consume large amounts of power relative to the power produced by the DMFC stack reducing the overall efficiency.
Another example of an issue with traditional DMFC systems relates to the thermal management. Typically, the thermal management is controlled by both the anode and the cathode stream. The cathode side cooling is achieved by cooling of the stack by means of the water vaporization by the air flowing through the stack. The cathode side cooling takes advantage of the high stoichiometric ratios (SR ranging from 4 to 6) and air flow rates flowing through the cathode for evaporating the water present in the cathode. The water evaporation in turn results in cooling of the stack. The exiting air saturated with water is then passed through a condenser system for the cathode side to condense the water and recycling it for replenishing the water in the anode feed. The anode side cooling is achieved by means of cooling the methanol and water mixture after it exits from the stack. This cooling radiator placed at the anode exit stream cools and condenses the liquid (methanol and water) and thus separates it from the carbon dioxide. This traditional approach for thermal management requires voluminous equipment that consumes a significant amount of power produced at the fuel cell stack for their operation and tends to reduce the overall system efficiency and system power density.
Additionally, to have a commercial fuel cell system that is water autonomous, neat or commercially available methanol should be the only fuel fed to the fuel cell. However, the neat methanol fuel needs to be strongly diluted in-situ in a bulky methanol-water mixing tank to reduce the methanol crossover across the membrane electrolyte due to concentration gradients. These problems are traditionally being addressed by either trying to develop a membrane that would restrict methanol and water permeation or by employing bulky and power consuming equipment (condensers, mixing tank, cooling fans for the condenser and heat and mass exchangers) for recycling water back to the anode from the cathode outlet stream. Due to the lack of a suitable membrane that could restrict water and methanol crossover the latter option is the preferred option. However, this approach leads to low power density as well as huge parasitic power consumption from multiple components and sub-systems constituting the balance of plant or auxiliary systems in a DMFC.
Accordingly, there is a need to develop new systems in particular related to water management that could be integrated with a direct methanol fuel cell stack and system.